1. Field of the Invention
The present invention relates to a zoom lens system.
2. Description of the Related Art
In recent years, a digital camera has received considerable attention as the next-generation camera instead of a 35 mm film camera. Furthermore, the camera has several categories in a broad range from a multi-functional type for business use to a portable popular type. The present invention especially pays attention to the portable popular type of category, and aims to provide a technology of realizing a thin video camera and a thin digital camera while securing high image qualities.
Thinning of the camera in a depth direction is hampered most by a thickness of an optical system, especially a zoom lens system from the surface closest to an object side to an image pickup surface. In recent years, it has been a mainstream to use a so-called collapsible lens barrel so that the optical system is projected from a camera body during photographing and stored in the camera body during carrying.
To realize the thinning and miniaturization, an image sensor may be miniaturized. However, to obtain the same number of pixels, pixel pitches need to be reduced. In this case, the image sensor has an insufficient sensitivity. This has to be covered by the optical system. When the pixel pitches decrease, the image quality is adversely affected by deterioration of a resolution due to diffraction. This also has to be covered by the optical system. Therefore, a bright optical system having a small F-number is required. Furthermore, to satisfy a demand that a user desires to enjoy a broad range of photographing, a zoom lens system having a large angle of field in a wide-angle end and having a large zoom ratio is demanded.
Examples of a comparatively compact zoom lens system having a zoom ratio which is as high as about threefold and having a large angle of field are disclosed in Japanese Patent Application Laid-Open Nos. 2004-318099, 2004-318106 and 2004-318107.
Each of these zoom lens systems has, in order from an object side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a positive refractive power. An axial space between lenses constituting the first lens unit is reduced, the first lens unit is constituted of only one negative lens, or an inner focusing system is adopted. In consequence, a zoom lens system having a comparatively compact lens barrel when collapsed is realized.